It has become the standard practice in eye surgery to use laser systems to assist in the individual steps of a surgical process. The interaction of the laser with the eye tissue, by which the tissue can be separated, replaces the formerly normal incisions with a scalpel. The laser systems are thus a component of ophthalmological therapy systems.
Femtosecond (fs) laser systems are frequently used for making incisions in eye tissue. In particular, such femtosecond laser systems are also used to make incisions in lens tissue afflicted with cataracts. For a portion of the laser cataract surgery, the surgeon requires direct access to the lens of the patient via the cornea, i.e. through the corneal tissue. This access can likewise be obtained using a laser beam. Such access incisions in the eye must fulfill various constraints in eye operations. On one hand, they must be small, such that the risk of infection is reduced. On the other hand, the access incision should be placed such that the aqueous humor cannot drain out. In particular, drainage must be prevented even under the influence of direct external mechanical effects. An important thing here is the fact that following such an incision, only the thin corneal epithelium heals. In the first days and weeks following the operation, the thick corneal stroma tissue is only “glued” together by adhesion forces. The stroma tissue is the middle corneal layer, and forms ca. 90% of the overall cornea thickness. For this reason, the wound may open due to external effects, e.g. pressure applied to the eyeball.
Complex, discontinuous corneal access incisions are difficult or impossible to execute with a scalpel. For this reason, it is standard practice to execute these incisions with a laser. However, known solutions can only obtain relatively simple incision geometries. FIG. 1a shows a currently typical bi-planar corneal access incision 1, i.e. an access incision 1 along two planes. Such corneal access incisions 1 as those shown in FIG. 1a have the disadvantage, however, that they lose their sealing properties when the eye is subjected to external mechanical effects, such that aqueous humor can drain out of the eye, or germs can penetrate, as is depicted in FIG. 1b, showing the effects of an external pressure 2 on the eyeball: An external pressure 2 results in an increase in the internal pressure in the eye (intra-ocular pressure IOP) 3. With an access incision 1 according to FIG. 1a, this results in turn in a leak 4, through which the aqueous humor can then escape. This problem persists long after the operation.
In the publication by David W. Langerman, “Architectural design of a self-sealing corneal tunnel, single hinge incision” (J Cataract Refract Surg 20, 1994), a special incision geometry for a corneal access incision 1 is presented. It differs from the incision geometry of the conventional corneal access incisions: The corneal access incision 1 of Langerman comprises, in addition to a bi-planar incision 11 in the corneal tissue 6 of the eye, first starting from an outer boundary surface 61 at approximately a right angle, then running in a suitable plane to an inner boundary surface 62, consequently forming a “tunnel,” an additional deep vertical partial incision 12, which, starting from the bi-planar incision 11, extends toward the inner boundary surface 62, without reaching it, however, see FIG. 2a. This additional vertical partial incision 12 results in the formation of a wedge-shaped valve flap with a hinge 5, close to the inner surface of the cornea. This incision is made manually, according to Langerman.
With a punctiform pressure 2 in the region of the outer “tunnel entrance” of the access incision 1 formed as a tunnel, composed of the bi-planar incision 11 and the deep vertical partial incision 12, the deformation and thus the opening of the tunnel can no longer continue along the floor of the tunnel to the inner “tunnel exit” and thus open the “corneal valve,” allowing aqueous humor to drain out of the eye. Instead, the wedge-shaped valve flap then swings upward on the hinge 5, and is pressed against the roof of the tunnel by the intra-ocular pressure 3. The result is an improvement in the “deformation stability” of the incision. FIG. 2b shows how such an access incision behaves when subjected to moderate external forces. The hinge 5 increases the tissue flexibility, such that the bonded corneal tissue 6 does not separate, and the access wound thus remains closed. This incision geometry thus offers a decisive medical advantage.
With higher pressure, however, this corneal access incision does not remain sealed. The “raising” of the wedge-shaped valve flap on the hinge, and thus the closing of the tunnel, is strongly limited by the space along the vertical partial incision and by the elasticity of the corneal tissue.
Furthermore, such an incision geometry did not become established in the past because it was very difficult to implement with a scalpel due to the limited thickness of the cornea.